The invention relates to a process and device for the frequency conversion of video signals, utilizing a dynamic calculation of the interpolation phases.
Known frequency conversion devices are utilized, among other things, for standards conversion, for example the conversion of a PAL or SECAM 50 Hz signal into an NTSC 60 Hz signal. They are also utilized to increase the scanning frequency of television receivers, typically from 50 Hz to 100 Hz, for the purpose of improving the quality of the picture, by reducing the large area flicker.
In the general case, the frequency of the signal to be converted and the frequency of the converted signal are known in advance. The interpolation process consists in creating intermediate fields between two reference fields of the input signal to be converted. This process is shown diagrammatically in FIG. 1.
The fields referenced 1 and 2 correspond respectively to the previous reference field of the input video sequence and to the next reference field of this sequence.
The field referenced 3 is the intermediate field to be created. This field is located temporally between the previous field and the next field, at an instant defined by a value xcex1 corresponding to the interpolation phase.
If tprev and tnext correspond to the instants relating to the previous field and to the next field, the instant tinterp relating to the intermediate field to be interpolated is then:                               T          interp                =                              t            prev                    +                      α            ⁡                          (                                                t                  next                                -                                  t                  prev                                            )                                                              =                              t            next                    -                                    (                              1                -                α                            )                        ⁢                          xe2x80x83                        ⁢                          (                                                t                  next                                -                                  t                  prev                                            )                                          
In a known manner, the interpolation processing can call upon linear filtering utilizing spatio-temporal filters with finite impulse response, or upon motion-compensated interpolation.
The processing methods utilized require knowledge of the interpolation phase for each field to be interpolated.
In the case where the input or output signal is not bound to a standard defining its frequency, a conversion device specific to these signals and in particular to their scanning frequency characteristic must be designed. This specificity naturally generates high costs. Moreover, modification of the scanning characteristics of a signal renders such a device unutilizable or, at the least, requires either manual intervention to render it compatible with the modified signal or signals, or the presence of circuits specific to these new characteristics.
The aim of the invention is to alleviate the aforesaid drawbacks.
To this end, the invention relates to a process for the field or frame frequency conversion utilizing a dynamic calculation of the interpolation phases, of an input video signal at a frequency F1 defined by a synchronization signal S1 into an output video signal at a variable frequency F2 defined by a synchronization signal S2, comprising a writing of the input signal to a memory on the basis of a write pointer PW-IN, and a reading, from the memory, to obtain the output signal, characterized in that it stores the position of the write pointer PW on receipt of the synchronization signal S1 so as to provide a value PW-IN, and on receipt of the synchronization signal S2 so as to provide a value PWxe2x80x94OUT, in that it dynamically calculates an interpolation phase xcex1 such that:       α    =                                                      P                              W                ⁢                                  xe2x80x83                                ⁢                _OUT                                      -                          P                              W                ⁢                                  xe2x80x83                                ⁢                _IN                                                          Δ            ⁢                          xe2x80x83                        ⁢                          P              W_FIELD                                      ⁢                  xe2x80x83                ⁢        if        ⁢                  xe2x80x83                ⁢                  P          W_OUT                    ≥              P        W_IN              or      α    =                                                      P              W_OUT                        +                                                            N                  CAP                                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              P                W_FIELD                                      -                          P              W_IN                                            Δ            ⁢                          xe2x80x83                        ⁢                          P              W_FIELD                                      ⁢                  xe2x80x83                ⁢        if        ⁢                  xe2x80x83                ⁢                  P                      W_OUT            ⁢                          xe2x80x83                                           less than               P        W_IN            
xcex94PWxe2x80x94FIELD being the memory quantity required for the storage of a field or frame of the input signal and NCAP the capacity of the memory expressed in terms of number of fields or frames,
and in that it performs a phase interpolation corresponding to this value.
According to a particular characteristic, three or four successive fields or frames are simultaneously stored in a memory and the interpolation is performed on the fields or frames preceding that being stored upon receipt of the signal S2 utilized for the calculation of the interpolation phase, on the basis of this calculated interpolation phase.
According to another characteristic, the calculation of the interpolation phase takes into account a shift of the write pointer corresponding to the duration of the field or frame blanking signal before the active video.
The invention also relates to a dynamic frequency conversion device for converting an input signal at a frequency F1 defined by a synchronization signal S1 into an output signal at a variable frequency F2 defined by the receipt of a synchronization signal S2, comprising a memory for writing the input signal on the basis of a write pointer PW, an interpolation circuit receiving the outputs from the memory for read-access to a preceding field or frame T1 and read-access to a next field or frame T2, characterized in that it also comprises:
a circuit for storing the pointer PW on receipt of the signal S1 so as to provide a value PWxe2x80x94IN,
a circuit for storing the pointer PW on receipt of the signal S2 so as to provide a value PWxe2x80x94OUT,
a circuit for calculating interpolation phase       α    =                                                      P                              W                ⁢                                  xe2x80x83                                ⁢                _OUT                                      -                          P                              W                ⁢                                  xe2x80x83                                ⁢                _IN                                                          Δ            ⁢                          xe2x80x83                        ⁢                          P              W_FIELD                                      ⁢                  xe2x80x83                ⁢        if        ⁢                  xe2x80x83                ⁢                  P          W_OUT                    ≥              P        W_IN              or      α    =                                                      P              W_OUT                        +                                                            N                  CAP                                ·                Δ                            ⁢                              xe2x80x83                            ⁢                              P                W_FIELD                                      -                          P              W_IN                                            Δ            ⁢                          xe2x80x83                        ⁢                          P              W_FIELD                                      ⁢                  xe2x80x83                ⁢        if        ⁢                  xe2x80x83                ⁢                  P                      W_OUT            ⁢                          xe2x80x83                                           less than               P        W_IN            
xcex94PWxe2x80x94FIELD being the memory quantity required for the storage of a field or frame of the input signal and NCAP the capacity of the memory expressed in terms of number of fields or frames,
and in that the interpolation circuit is coupled to the calculating circuit so as to receive this interpolation phase and calculate the interpolated field or frame as a function of the interpolation phase.
By virtue of the invention, the process automatically calculates the interpolation phase corresponding to the output signal to be generated, when the scanning frequency of this output signal is not known a priori.
The device obtained is compatible with any type of output signal, at least in a wide frequency range. The device is simple to implement and cheap. It makes it possible to supply adjustable scanning frequency apparatuses, such as monitors of PC-type personal computers.